This research aims to study motor units having different axonal conduction velocities with respect to the number of muscle fibres innervated, their cross sectional areas and specific tensions (kg/cm2). The data should indicate which of these variables is responsible for the precise and important relation between axonal size and force developed in fatigue-resistant motor units. In such units log 10 maximal tetanic tension shows a nearly linear relation to axonal conduction velocity. Single motor axons to superficial lumbrical muscle of cat will be isolated in ventral root, conduction velocity determined and muscle fibres of a unit labeled with C14 2-deoxyglucose during activity. Using superficial lumbrical which has 2 heads, comparisons between pairs of units can be made in single animals, avoiding scatter in data introduced by interanimal variation. Numbers and diameters of labeled fibres will be measured; specific tension will be derived from maximal tetanic tension divided by cross sectional area of unit's fibres. Similar measurements will be made after disuse (tenotomy) to determine which variables are altered. This is relevant to clinical conditions in which disuse leads to profound changes in muscle. Studies will also be made in muscles reinnervated after nerve crush to determine fibre numbers, areas, specific tensions and spatial distributions. This has clear implications for clinical problems of nerve injury and reinnervation.